Moderate power, broadband projectors find application in deep-towed seismic profiling and underwater acoustics research. Current projectors make use of hydraulic or pneumatic devices, electric spark sources, and electromagnetic devices. Free-flooding piezoelectric ceramic tubes are often employed as drivers for underwater sound projectors because their characteristics are essentially unaffected by depth; the use of such tubes as projectors has been described in a paper by J.B. Lee entitled "Low-Frequency Resonant-Tube Projector For Underwater Sound," presented at the OCEANS '74 conference.
Short tubes, or rings, in which the fundamental cavity mode is closely coupled to the ceramic ring mode, can deliver high acoustic power over a wide frequency band. However, because the power output of a simple acoustic source is proportional to the squares of volume velocity and frequency, projectors at low frequency often depend on large vibrating areas in order to achieve reasonable power outputs; thus, even for some low frequency applications that require only moderate power, the projector can become unacceptably large and expensive. Long tubes, or pipes, on the other hand, exhibit a number of narrow-band cavity resonances at which low frequency sound is radiated efficiently, but the output is usually negligible between these resonances. Decreasing the response of certain resonances and increasing the response between certain other resonances enables a piezoelectric ceramic tube to be used over a wide band of frequencies, while retaining the advantages of a relatively high electrical-to-acoustical efficiency, a medium to high power output over the operating frequency band, and the absence of depth dependency and depth limitations.